The manufacture of steel shot and grit heretofore has been basically standard within the steel abrasives industry and steel shot and grit is sold subject to either the Society of Automotive Engineers (SAE) standards or the Steel Founders Society of America (SFSA) standards. The SFSA specification for cast steel abrasives carries the designation 20T-66 and is widely used. It sets forth technical requirements, particularly with regard to chemistry, microstructure, appearance and hardness. The process of manufacturing typically includes steps of: charging, melting, refining and pouring, with the pouring step being accomplished by directing the melted and refined molten steel through a stream of water under pressure and into a water quenching bath. The as-cast shot then, after being dried, is taken to a hardening furnace which is typically a continuously rotating retort having a series of gas burners controlling its temperature. After going into a quenching pit from the hardening furnace and again going through a drying operation, both of which consume natural gas, the hardened shot is subjected to a tempering operation to make tempered martensitic shot or to a crushing operation to make grit. The tempered martensitic shot ends up with a preferred hardness on the Rc Scale of from about 40 to 50.
As a direct result of the recent "energy crisis" and resulting reduction of allocations of natural gas to industrial users, attempts to determine ways to minimize consumption of natural gas in the steel abrasives manufacturing processes led to experimentation with close carbon control and boron additions. The fact that boron additions in steel generally could improve hardenability was known and a typical discussion of this phenomena was published in "Foote Notes on steel, No. 2", Information of interest to the steel industry, Published by Foote Mineral Company, Exton, Pa. 19341. Another discussion can be found in the Metals Handbook, Vol. 1, 8th Ed. (ASM); in particular, the section on "Selection for Hardenability", beginning at page 189 and FIGS. 1, 6(b) and 13 of that section. Metallurgists, however, have generally thought that the addition of boron has the effect of making coarse grain size and it was not known whether this would be controllable in steel abrasives. Other problems which could have occurred by adding boron to steel shot heats relate to the SFSA 20T-66 shot internal and external appearance standards which limit the permissible amount of voids, shrinkage and cracks. The invention is the culmination of work which included many heats with various carbon percents by weight and various addition of levels of ferro-boron, including boron free heats. Typically, the heats were 7 tons in magnitude and were made in a 9 foot "Whiting" 3500 KVA direct arc electric furnace under the same conditions.